Radio recombination lines (RRLs) in ultra-compact H II Regions have intrinsic widths (FWHM) of about 20 to 50 km/s. However, if there are significant bulk motions of the ionized gas, the lines may be 80 km/s wide (e.g., G5.89-0.39). Electron pressure broadening can be significant at frequencies below about 8 GHz, and the line wings of the Voigt profile can extend to 200 km/s or more. Thus, a total bandwidth of over 300 km/s is necessary to provide minimal spectral baseline, to determine the line profile, and to measure the continuum level. The line-to-continuum ratio is a critical parameter in any model of RRL emission and it is important to measure the continuum under the same conditions as the line.
Every H line is accompanied by the corresponding He, C and X lines which are spaced at 121 km/s and 150 km/s from the H line (due to the differences in their atomic weights). The He line is typically 10% the intensity of the H line. The C line, is weaker still and much more narrow ( km/s FWHM). All of the lines provide important information about the physical conditions of the ionized gas and the gas excitation and dynamics. The He line yields the He abundance which is an important diagnostic of the chemical evolution of the Galaxy. The C line is a new way to probe the excitation of photodissociation regions around the H II regions of high and intermediate mass stars. It is not possible to cover all these lines in one band with the present correlator. Ideally one would like to cover a velocity range of 400-500 km/s. At 7mm this requires a spectral line band width of MHz. For sensitivity one would like to obtain the RCP and LCP data simultaneously. To resolve the narrow C line a channel spacing of 1 km/s is needed. Thus, more than 500 channels are needed over an approximately 70 MHz band with two polarizations.
Very broad ( km/s) and in some cases quite asymmetric RRLs have been observed from ultra-compact H II regions. These observations, which must be made at the highest VLA frequencies where the H II regions are optically thin, have been limited in several respects. First, in order to measure the continuum level, very large bandwidths are required. The current (recirculating) correlator severely limits the number of available channels when large bandwidths are used. Additionally, high spectral resolution is required to resolve lines which may be blended. Secondly, as discussed above, a large portion of the continuum emission from ultra-compact H II regions is very weak and extended, thus producing very weak RRL emission. Increased continuum sensitivity and increased surface brightness sensitivity would allow us to determine the kinematics of the ionized gas over the entire extent of the H II regions.